1. Field of the Invention
This invention relates to a chiral smectic liquid crystal composition. More particularly it relates to a ferroelectric liquid crystal composition having a long helical pitch and superior response properties, obtained by adding a ferroelectric chiral smectic liquid crystal to at least one kind of liquid crystal compounds of a directly linked ring type structure having a smectic C phase (hereinafter abbreviated to S.sub.c phase).
2. Description of the Prior Art
Liquid crystal compounds have been widely used as a material for display elements, but most of such liquid crystal display elements are of TN type display mode and as the liquid crystal materials, those belonging to a nematic phase have been used.
Since the TN type display mode is of non-emissive type, it has specific features that eyes are not tired and the electric power consumption is very small, whereas it has drawbacks that response is slow and the display is not seen in certain angles of view. Recently, the display mode has come to be turned toward a direction of making use of the specific feature in the form of a flat display. In particular, high speed response properties and a broad angle of view have been required. Thus, in order to satisfy such a requirement, improvement in liquid crystal materials has been attempted. However, as compared with other emissive type displays such as electroluminescence display, plasma display, etc., the TN display mode has been confirmed to be considerably inferior in the responce time and the broadness of the angle of view to the above emissive type displays. Making use of the specific features of the non-emissive type and the low power consumption of the liquid crystal display elements, and yet in order to ensure the response properties corresponding to those of the emissive type display elements, it has been indispensable to develop a new liquid crystal display mode. As one of such attempts, a display mode utilizing the light switching phenomenon of ferroelectric liquid crystals has been proposed by N. A. Clark and S. T. Lagerwall (see Applied Physics Letters, Vol. 36, page 899 (1980)). Ferroelectric liquid crystals have been announced by R. B. Mayer for the first time (see Journal de Physique Vol. 36, page 69 (1975)), and in the aspect of crystal structure, the crystals belong to chiral smectic C phase, chiral smectic I phase, chiral smectic F phase, chiral smectic G phase and chiral smectic H phase (hereinafter abbreviated to S.sub.c * phase, S.sub.I * phase, S.sub.F * phase, S.sub.G * phase and S.sub.H * phase, respectively).
When the light switching effect of S.sub.c * phase is applied to display elements, there are two superior specific features as compared with TN display mode. The first specific feature consists in that the response is effected at an extremely high speed and the response time is 1/100 or less of that of TN display elements. The second specific feature consists in that there is a memory effect and multiplexing drive is easy in cooperation with the above high speed response properties.
As the display mode, two modes are considered. One mode is of a birefringence type using two polarizers and the other is of guest-host type using a dichroic dyestuff. S.sub.c * phase has a spontaneous polarization; hence by inverting the polarity of impressed voltage, liquid crystal molecules turn over by .pi.-rotation around their helical axes as the rotating axes. The liquid crystal display mode using the ferroelectric chiral smectic liquid crystals is a mode utilizing the above reverse phenomenon of liquid crystal molecule. The birefringence type display mode is a mode utilizing a fact that the liquid crystal molecule has a birefringent index and behaves in the same manner as in the case of optically uniaxial crystal. In the case of this mode, by filling a liquid crystal composition having a S.sub.c * phase in a liquid crystal display cell subjected to an aligining treatment so that the liquid crystal molecules can be aligned in parallel to the surface of the electrodes; placing the liquid crystal cell between two sheets of polarizers arranged so as to form crossed nicols state, so that the director of liquid crystal molecules can be in parallel to the polarization plane of one of the polarizers; and impressing a voltage to invert the polarity, the director of liquid crystal molecules is arranged in the direction of extinction position or diagonal position, whereby it is possible to obtain a dark view or a bright view.
Since liquid crystal molecules are rotated around the helical axis by the polarity reversal through impressed voltage, it is preferred in the case of the birefringence type display mode that the angle of the major axis of liquid crystal molecule made against the helical axis (hereinafter referred to as "tilt angle") be 22.5.degree..
On the other hand, in the case of the guest-host type, it is possible to obtain an uncolored view or a colored view, depending on the angle of the direction of the transition moment of a dichroic dyestuff dissolved in a liquid crystal compound or a liquid crystal composition, made against the polarization plane of the polarizer. When the direction of the transition moment of the dichroic dyestuff molecule and the direction of the polarization plane are in parallel to each other in the case of one polarity, and the former direction is crossed with the latter direction in the case of the other polarity, then the best contrast is obtained. Namely, when the tilt angle of liquid crystal molecule is 45.degree., it is possible to obtain the best contrast.
In order that the display element using a ferroelectric chiral smectic liquid crystal can have a memory, it is regarded as necessary to be able to form two stable states (bistable state) on the surface of electrode by polarity inversion of impressed voltage. In order to obtain a liquid crystal display element having the above bistable state and high speed response properties, it is necessary to make the cell gap d less than the helical pitch P (d&gt;P) to untwist the helix, as proposed by N. A. Clark et al (see N. A. Clark et al, Applied Physics Letters, vol. 36, page 899 (1980)).
Further, when the display element using a ferroelectric liquid crystal is brought into the above bistable state, there is generally the following relationship (1) among the threshold voltage Ec required for inverting molecule, the helical pitch P and the value of the spontaneous polarization Ps: ##EQU1## (see R. B. Mayer, Molecular Crystal and Liquid Crystal, vol. 40, page 33 (1977)).
In the above formula (1), K represents a torsional elastic constant at the time of deformation due to turning-over of molecule. As apparent from the equation (1), in order to reduce the threshold voltage, a longer helical pitch and a larger spontaneous polarization value are preferred.
Further, there is the following relationship among the response time (.tau.), the spontaneous polarization value (Ps) and the viscosity (.eta.): ##EQU2##
In the above equation (2), E represents an impressed voltage. Thus, in order to raise the response speed, there are the following methods:
(a) the viscosity .eta. is reduced; PA1 (b) the spontaneous polarization value Ps is increased; and PA1 (c) the impressed voltage E is elevated.
Liquid crystal cells are driven by means of LSI (Large Scale Integrated Circuit) which is generally operated under a considerably low voltage. Thus the impressed voltage of liquid crystal cells is preferred to be as low as possible. Accordingly, it is actually necessary to lower the viscosity .eta. or raise the spontaneous polarization value Ps.
In general, ferroelectric chiral smectic liquid crystal compounds having a large spontaneous polarization value, have a tendency that their viscosity increases. Thus, even if the spontaneous polarization value is increased uselessly, the viscosity increases accordingly, and as a result, it is considered that the response speed is not raised so much.
As described above, in order to obtain an element having a high response speed by applying the light-switching effect of S.sub.c * phase, a ferroelectric chiral smectic liquid crystal composition having a low viscosity is required.